The storage, handling, and transfer of HLW, such as spent nuclear fuel, requires special care and procedural safeguards. In the operation of nuclear reactors, hollow zircaloy tubes filled with enriched uranium, known as fuel assemblies, are burned up inside the nuclear reactor core. It is customary to remove these fuel assemblies from the reactor after their energy has been depleted down to a predetermined level. Upon depletion and subsequent removal, this spent nuclear fuel (“SNF”) is still highly radioactive and produces considerable heat, requiring that great care be taken in its subsequent packaging, transporting, and storing. Specifically, the SNF emits extremely dangerous neutrons and gamma photons. It is imperative that these neutrons and gamma photons be contained at all times subsequent to removal from the reactor core.
In defueling a nuclear reactor, it is common place to remove the SNF from the reactor and place the SNF under water, in what is generally known as a spent fuel pool or pond store. The pool water facilitates cooling of the SNF and provides adequate radiation shielding. The SNF is stored in the pool for a period long enough to allow the decay of heat and radiation to a sufficiently low level to allow the SNF to be transported with safety. However, because of safety, space, and economic concerns, use of the pool alone is not satisfactory when the SNF needs to be stored for a considerable length of time. Thus, when long-term storage of SNF is required, it is standard practice in the nuclear industry to store the SNF in a dry state subsequent to a brief storage period in the spent fuel pool, i.e., storing the SNF in a dry inert gas atmosphere encased within a structure that provides adequate radiation shielding. One typical structure that is used to store SNF for long periods of time in the dry state is a storage cask.
Storage casks have a cavity suitably sized to receive a canister of SNF and are designed to be large, heavy structures made of steel, lead, concrete and an environmentally suitable hydrogenous material. Typically, storage casks weigh about 150 tons and have a height greater than 15 ft. A common problem associated with storage casks is that they are too heavy to be lifted by most nuclear power plant cranes. Another common problem is that storage casks are generally too large to be placed in spent fuel pools. Thus, in order to store SNF in a storage cask subsequent to being cooled in the pool, the SNF must be removed from the pool, prepared in a staging area, and transported to the storage cask. Adequate radiation shielding is needed throughout all stages of this transfer procedure.
As a result of the SNF's need for removal from the spent fuel pool and additional transportation to a storage cask, an open canister is typically submerged in the spent fuel pool prior to the SNF being removed from the reactor core. The SNF is then placed directly into the open canister while submerged in the water. However, even after sealing, the canister alone does not provide adequate containment of the SNF's radiation. A loaded canister cannot be removed or transported from the spent fuel pool without additional radiation shielding. Thus, apparatus and methods that provide additional radiation shielding during the transport of the SNF have been developed. The additional radiation shielding is typically achieved by positioning the canisters in large cylindrical containers called transfer casks while submerged within the pool. Similar to storage casks, transfer casks have a cavity suitably sized to receive the canister of and are designed to shield the environment from the radiation emitted by the SNF within.
In facilities utilizing transfer casks to transport loaded canisters, an empty canister is first placed into the cavity of an open transfer cask. The canister and transfer cask are then submerged in the spent fuel pool. Previously discharged SNF from reactors located in wet storage is moved into the submerged canister (which is within the transfer cask and filled with water). The loaded canister is then fitted with its lid, enclosing the SNF and the water from the pool within the canister. The loaded canister and transfer cask are then removed from the pool by a crane and set down in a staging area to prepare the SNF-loaded canister for storage or transportation in a dry condition. In order for an SNF-loaded canister to be properly prepared for dry storage or transportation, the United States Nuclear Regulatory Commission (“NRC”) requires that the SNF and interior of the canister be adequately dried before the canister is sealed and transferred to the storage cask. Specifically, NRC regulations mandate that the vapor pressure (“vP”) within the canister be at or below 3 Torr (1 Torr=1 mm Hg) before the canister is backfilled with an inert gas and sealed. Vapor pressure is the pressure of the vapor over a liquid at equilibrium, wherein equilibrium is defined as that condition where an equal number of molecules are transforming from the liquid phase to gas phase as there are molecules transforming from the gas phase to liquid phase. Requiring a low vP of 3 Torr or less assures an adequately dry space in the canister interior suitable for long-term SNF storage or transportation.
Currently, nuclear facilities comply with the NRC's 3 Torr or less vP requirement by performing a vacuum drying process. In performing this process, the bulk water that is within the canister is first drained from the canister. Once the bulk of the liquid water is drained, a vacuum system is coupled to the canister and activated so as to create a sub-atmospheric pressure condition within the canister. The sub-atmospheric condition within the canister facilitates evaporation of the remaining liquid water while the vacuum helps remove the water vapor. The vP within the canister is empirically ascertained through a vacuum-and-hold procedure. If necessary, the vacuum-and-hold procedure is repeated until the pressure rise during a prescribed test duration (30 minutes) is limited to 3 Torr. Once the vacuum drying passes the acceptance test, the canister is backfilled with an inert gas and the canister is sealed. The transfer cask (with the canister therein) is then transported to a position above a storage cask and the SNF-loaded canister is transferred into the storage for long-term storage.
Current methods of satisfying the NRC's 3 Torr or less vP requirement are time consuming, manually intensive and prone to error from line and valve leakages. Any time the canister must be physically approached for vacuum monitoring and dryness testing, there is the risk of exposing the work personnel to high radiation. Moreover, the creation of sub-atmospheric conditions in the canister requires expensive vacuum equipment and can cause complicated equipment problems.